Heat-sensitive sheet for stencil printing

ABSTRACT

Disclosed is a heat-sensitive sheet for stencil printing comprising a laminate of a thermoplastic resin film and a porous support mainly composed of synthetic fibers, the sheet having a wet tensile strength in the machine direction of 200 gf/cm or higher, a KES bending rigidity B value in the machine or cross direction of 0.02 g·cm 2 /cm or higher, and a PPS smoothness determined when a film is pressed against the surface of the porous support of the sheet of 0.9 μm or higher; the sheet has an excellent runnability, occurrence of wrinkles on the sheet when it is wound around a drum of a stencil printing machine is efficiently prevented, and elongation of the sheet is repressed and occurrence of wrinkles on the sheet can be prevented even when a large number of paper sheets are printed, and thus printed paper sheets having sharp images excellent in reproducibility of a manuscript can be provided.

TECHNICAL FIELD

The present invention relates to a heat-sensitive sheet for stencilprinting. More specifically, the invention relates to a heat-sensitivesheet for stencil printing which causes no jamming in a stencil printingapparatus during feeding and no wrinkle at the time of winding around orloaded on a printing drum, and is not elongated even when a large numberof sheets of paper are printed, and thus sharp printed images can beobtained.

(For the purpose of the present invention, the words “sheets of paper”are sometimes referred to as “papers” and the words “sheet for stencilprinting” are sometimes condensed to “stencil sheet” or furtherabbreviated to “the sheet” for brevity.)

BACKGROUND ART

Heretofore, a heat-sensitive sheet for stencil printing was notnecessarily satisfactory in definition or sharpness of printed images,particularly in evenness of its solid parts. While various causes can beadduced for such circumstance, a condition ascribable to the fiberswhich constitute the support can be mentioned as one of the causes.

That is, a tissue paper (thin paper) which is conventionally used as theporous support in a stencil sheet and composed of natural fibers hasdefects as follows:

Passing of ink is liable to become uneven since diameter of the fibersis comparatively large and uneven, and cross section of the fibers isflat. Particularly, passing of ink is obstructed by the fibers locatedat directly under perforated portions to cause fading (or blurring) ofprinted letters, and

Smoothness of the surface of a film laminated on the support isdeteriorated by the thick fibers, and contact of the film with a thermalhead at the time of perforations is poor to often causes deficientperforations, thus, voids are formed in solid printing.

In order to improve these defects, such countermeasures have beenproposed that a paper prepared by using a blend of natural (cellulosic)fibers and synthetic fibers such as polyester fibers through a wetpapermaking process or a non-woven fabric is used instead of the abovementioned tissue paper composed only of natural fibers to make thefibers in the porous support fine on average or to reduce the basisweight of the paper or fabric (Laid-open Japanese Patent PublicationNos. Sho 59-2896, Sho 59-16793, and Hei 2-67197).

Definition of images is improved by thinning the diameter of fibers inthe porous support or reducing the basis weight of the paper or fabric.In this case, however, there occur such problems that runnability of thesheet is lowered to cause jamming in the printing apparatuses duringfeeding, and that wrinkles occur on the sheet when an unperforated orperforated sheet is wound around and loaded on a printing drum, and thewrinkles degrade the quality of printing. Besides, there is a defectthat the sheet is elongated (elongation at printing) or wrinkles occuron the sheet (wrinkles at printing) and thus reproducibility of amanuscript in printed papers is lowered when a number of papers areprinted.

Further, in order to improve these defects, a stencil sheet having aspecific tensile strength and flexural rigidity, that is, a specifiedtenacity and stiffness (Laid-open Japanese Patent Publication No. Hei8-67080), and another stencil sheet having a specific wet elongationwhen the sheets is stretched under a certain load (Laid-open JapanesePatent Publication No. Hei 5-104875) have been proposed.

However, even when the sheet has the strength and rigidity (stiffness)described above, the effect of preventing the wrinkle occurrence whenthe sheet is wound around a printing drum is low, and the sheet is notthoroughly satisfied even as to the wrinkle occurrence at printing.

DISCLOSURE OF THE INVENTION

An object of the present invention is to solve the problems in the priorart described above. Another object of the present invention is toprovide a heat-sensitive sheet for stencil printing which sheet isexcellent in runnability, occurrence of wrinkles on the sheet when thesheet is wound around a drum of a printing apparatus is efficientlyprevented, elongation of the sheet is repressed and occurrence ofwrinkles on the sheet can be prevented when a large number of papers areprinted, and thus printed papers having a sharp images excellent inreproducibility of a manuscript can be obtained.

As a result of diligent investigations by the present inventors with theattention being focused on the mechanism by which wrinkles occur on thesheet when it is wound around a printing drum, the sheet is elongated,and wrinkles occur on the sheet at the time of printing, it has beenfound out that the problems described above can be solved by specifyingthe smoothness of the surface of porous support in the sheet and thestrength of the sheet, leading to the accomplishment of the presentinvention.

The present invention is concerned with a heat-sensitive sheet forstencil printing comprising a laminate of a thermoplastic resin film anda porous support mainly composed of synthetic fibers which sheet has awet tensile strength in the machine direction (longitudinal direction)of 200 gf/cm or higher, a KES bending rigidity B value in the machinedirection or cross direction (lateral direction) of 0.02 g·cm²/cm orhigher, and a PPS smoothness determined when a film is pressed againstthe surface of the porous support of the sheet of 0.9 μm or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the portion for perforating aheat-sensitive stencil sheet in a stencil printing apparatus.

FIG. 2 is a schematic illustration of the mechanism by which wrinklesare formed on the sheet when a perforated heat-sensitive stencil sheetis wound around a printing drum.

FIG. 3 is a schematic illustration of a part of printing portion in astencil printing apparatus.

FIG. 4 is a graph showing a M-K curve used in the determination of KESbending rigidity B value of a heat-sensitive stencil sheet.

Now, the mechanisms by which wrinkles occur on a stencil sheet when thestencil sheet is wound around a drum in a printing apparatus, thestencil sheet is elongated at printing, and wrinkles occur on thestencil sheet at printing are described with reference to drawings.

FIG. 1 is to schematically illustrate particularly the portion forperforating a stencil sheet in a stencil printing apparatus.

In a printing apparatus shown in FIG. 1, when a manuscript for printingis set at a read portion (not shown in the drawing), a read sensor readsthe light and shade corresponding to the diagrams and letters in themanuscript as digital signals, and the signals are transmitted tothermal head 10. On the other hand, stencil sheet 2 set on a holder isfed up to thermal head 10 with feeding roller 11, and then perforated bythe heat of thermal head 10 to form a perforated stencil sheet. Frontend portion of the perforated stencil sheet 2 is held by clamp portion 9and then wound around printing drum 1. Ink is squeezed out from theinside of printing drum 1 and transferred to a printing paper throughperforated portions of the sheet 2 to complete printing. Printing paperis fed in synchronization with the rotation of printing drum 1, and anecessary number of sheets are continuously printed.

In FIG. 1, 3 is a press roller and 13 is a cutter.

Mechanism by which wrinkles occur when stencil sheet 2 is wound aroundprinting drum 1 is described with reference to FIG. 2. In thisconnection, whereas printing drum 1 has a cylindrical shape in reality,it is shown in a developed state in FIG. 2 for the convenience ofexplanation.

Tension is applied on stencil sheet 2 toward the running direction in aprinting apparatus. Accordingly, when the strength of stencil sheet 2 isinsufficient or the sturdiness of the sheet is insufficient, adistortion occurs on a running stencil sheet, and thus wrinklessometimes occur on the sheet in the machine direction when the sheet iswound around a printing drum. Besides, even when the strength of stencilsheet 2 is sufficient and the sheet is stiff, wrinkles sometimes occurat the time when the sheet is wound on a printing drum as describedbelow.

That is, in FIG. 2, the front end of stencil sheet 2 is held by clampportion 9 to place the sheet on printing drum 1 ((A)). At this time,stencil sheet 2 is placed on printing drum 1 such that printing surface6, that is, the layer of a thermoplastic resin film becomes the uppersurface of the sheet. Subsequently, the stencil sheet 2 is wound aroundthe outer surface of printing drum 1 by the rotation of the printingdrum 1 while being pressed with press roller 3, and finally wholeprinting surface 6 is placed on the drum ((B), (C), (D)). While thesheet is wound around the printing drum, not-contact portion 5 at whichthe stencil sheet 2 does not contact with the printing drum and has aair layer between the stencil sheet 2 and printing drum 1 exists at theother end of the placed stencil sheet 2.

However, when stencil sheet 2 is wound around printing drum 1, airbubble 4 (actually small air bubbles) enter between the stencil sheet 2and the surface of printing drum 1, and air bubble 4 raises the stencilsheet 2 from the surface of printing drum 1. After the clamp portionpassed the position of press roller 3, the stencil sheet 2 is pressedwith press roller 3 to firmly adhere it on the surface of the drum whilebeing rotated. On the other hand, the raised portion (that is, airbubble 4 ) gradually becomes large as printing drum 1 rotates, moves tothe direction opposite to the rotation ((B₁)) towards not-contactportion 5. When the air bubble became large, sheet folding 7 occurs((C₁)). The folded portion forms wrinkles 8 as the air inside is purged((D₁)).

In such mechanism by which wrinkles are formed at a stage of winding asheet around a drum, the smaller the air bubble 4, the more hardlyoccurs a partial folding back of the stencil sheet 2.

On the other hand, in order to reduce the occurrence of a large airbubble, it is necessary to quickly purge small air bubbles from thespace between stencil sheet 2 and printing drum 1.

In order to quickly purge air bubbles, it is necessary to preventstencil sheet 2 from firmly sticking to the outer surface of printingdrum 1 with an ink. The sticking of the sheet becomes small and weakwith increase in the unevenness on the surface of a porous support ofstencil sheet 2 which contacts with the outer surface of printing drum1, that is, with decrease in the smoothness of the surface of the poroussupport.

Next, the cause of elongation of a stencil sheet at printing isdescribed with reference to FIG. 3. FIG. 3 is a schematic illustrationof a part of printing portion in a representative stencil printingapparatus. In FIG. 3, printing drum 1 is provided with clamp portion 9having fixed part 12 fixed at a part of the perimeter of the drum, andmovable part 14 which is connected to the fixed part by a hinge and canbe opened and closed with the hinge being its axis. They are designedsuch that stencil sheet 2 is fed with a sheet feeding device to openedclamp portion 9, and the front end of the sheet is clamped with clampportion 9. Below printing drum 1, pressing device 15 for printing paper20 is placed. Printing paper 20 is fed by the rotation of a pair ofpaper feeding rollers 21, 22 to a printing position at a predeterminedtiming, synchronized with a rotation of stencil sheet 2 placed onprinting drum 1 as shown by arrow C. Supporting arm 17 is moved up anddown with the axis 16 being its center by a member such as a cam (notshown in the drawing) for moving the arm up and down, synchronized withthe feeding of the paper, and the printing paper 20 can be pressedagainst the stencil sheet 2 by free rotation type press roller supportedby the arm to conduct printing. Printing drum 1 and pressing device 15are set up on a frame (not shown in the drawing).

However, since press roller 3 is not rotated at the moment when theroller was pressed to a stencil sheet in such printing portion, a forcein the direction opposite to that of the rotation of printing drum 1 isapplied on printing paper 20 and the stencil sheet 2 in turn as shown byarrow X to cause elongation of the stencil sheet 2. Further, whenrunning speed of printing paper 20 fed with paper feeding rollers 21, 22becomes slower than the rotational speed of printing drum 1, a backtension is applied on printing paper 20. Even such a condition becomes acause of producing elongation of stencil sheet 2.

When the smoothness of the surface of porous support of a stencil sheetis low, friction resistance between the porous support and the surfaceof a printing drum is generally small.

At the time of printing, a printing paper exerts the stencil sheet anexternal stress which is caused by the back tension of paper feedingrollers and acts as a force to elongate the stencil sheet, and a pressroller also exerts the sheet a resisting force, and thus the stencilsheet is elongated.

At this time, when the press roller parted from the printing drum toavoid a convex in a clamp portion, the stress and resisting force addedto the stencil sheet will vanish, and the elongated stencil sheet tendsto return to an original length on the printing drum as in the case ofelastic deformation.

However, when the friction resistance between the porous support and thesurface of the printing drum is large, the stencil sheet difficultlyreturns to the original length and remains nearly as elongated.

On the other hand, when the friction resistance is small, returning ofthe stencil sheet to the original length is large, and thus elongationof the stencil sheet can be repressed in the end.

Further, against a certain external stress, the higher the tensilestrength of a stencil sheet, the smaller is the elongation of the sheetat printing and the more hardly occurs wrinkles on the sheet.

Since the heat-sensitive stencil sheet of the present invention isadjusted, from the viewpoints described above, such that PPS smoothnessof the surface of the porous support contacting with a printing drum ishigher than a certain value and the sheet has a specific strength,occurrence of wrinkles on the sheet when the sheet is wound around aprinting drum can be prevented, elongation of the sheet when a largenumber of paper sheets are printed is repressed, occurrence of wrinklesat the time of printing can be prevented, reproducibility of diagramsand letters on a manuscript in printed papers is excellent, anddefinition of images in printed portions is not reduced.

In the present invention, PPS smoothness of the surface of the poroussupport in a heat-sensitive stencil sheet is 0.9 μm or higher,preferably 1.1 μm or higher, and more desirably 1.7 μm or higher.

In this connection, PPS smoothness means smoothness determined byair-leak method using Parker Print-Surf type paper smoothness tester(produced by Messmer Buchel Co. Ltd.), and PPS smoothness in the presentinvention means the value determined when a film having a thickness of0.1 to 10 μm and a PPS smoothness of 0.0 μm is pressed against thesurface of the porous support of a stencil sheet.

While the film used for determination of the smoothness is notspecifically limited as long as the film has a thickness mentioned aboveand a surface smoothness of 0.0 μm, usually a film of a thermoplasticresin used for producing the heat-sensitive stencil sheet is employed.As described above, when the PPS smoothness is determined, air leakagein the thickness direction of the porous support can be avoided bydetermining the smoothness while pressing a film against the surface ofa porous support, and thus the surface smoothness of the porous supportcan accurately be determined.

When PPS smoothness of the surface of the porous support in a stencilsheet is lower than 0.9 μm, smoothness of the surface of a poroussupport of a stencil sheet is high and the sheet is liable to stickfirmly to the surface of a printing drum, air bubbles occurred aredifficultly purged, and thus wrinkles are ready to occur on the sheetwhen the sheet is wound around a printing drum thereby printing qualityis degraded. Also, at the time of printing a large number of papers,elongation of a stencil sheet becomes large and thus reproducibility ofa manuscript in printed papers is lowered.

While the upper limit of PPS smoothness of the surface of the poroussupport is not specifically limited, it is desirably adjusted to belower than 8.0 μm from the viewpoint of the smoothness of the surface ofthe film of a stencil sheet.

In the present invention, KES flexural rigidity B value of a stencilsheet in the machine or cross direction is 0.02 g·cm²/cm or higher andpreferably 0.03 to 0.10 g·cm²/cm. When the KES flexural rigidity B valueis lower than 0.02 g·cm²/cm, stiffness of the stencil sheet isinsufficient, wrinkles are readily occur when the sheet is wound arounda printing drum and thus printing quality is degraded.

As used herein, the term “machine direction” means the running directionof a heat-sensitive stencil sheet when it is fed to a printing apparatusor the direction to which the sheet wound around a printing drum isrotated. The term “KES” also used in the present specification is anabbreviation for Kawabata's Evaluation System for Fabrics which systemwas devised by Professor Kawabata at Kyoto University and has widelybeen used as a method for determining hand feeling of woven or knittedfabrics in terms of a physical quantity.

In the present invention, wet tensile strength of a stencil sheet in themachine direction is 200 gf/cm or higher and preferably 300 gf/cm orhigher. When wet tensile strength of the sheet in the mechanicaldirection is lower than 200 gf/cm, strength of a stencil sheet isinsufficient and running of the sheet sometimes can not smoothly beconducted since a tension is applied on the sheet in its runningdirection in a printing apparatus. In an extreme case, tearing of thesheet occurs, or wrinkles sometimes occur on the sheet due toinsufficient strength of the sheet when the sheet is wound around aprinting drum.

Further, when a large number of papers are printed, elongation of astencil sheet at printing becomes large, reproducibility of a manuscriptin printed papers reduces, or wrinkles occur on the sheet at printing tocause a strained or unclear printing in the portions of wrinkles formedat printing, thereby reduces the definition of images on printed papers.

Heat-sensitive sheet for stencil printing of the present invention canbe obtained by laminating a thermoplastic resin film and a poroussupport mainly composed of synthetic fibers.

As the thermoplastic resin film used in the present invention, a knownfilm comprising a polyester, polyamide, polypropylene, polyethylene,polyvinyl chloride, polyvinylidene chloride, or their copolymer can beused, but a polyester film is especially preferable from the viewpointof sensitivity to perforation.

As the polyester used for the thermoplastic resin film, a polyethyleneterephthalate, copolymer of ethylene terephthalate with ethyleneisophthalate, polyethylene-2,6-naphthalate, polyhexamethyleneterephthalate, and copolymer of hexamethylene terephthalate with1,4-cyclohexanedimethylene terephthalate can be mentioned.

The thermoplastic resin film can be prepared by a known T-die extrusionmethod, inflation method, or the like, and a stretched film,particularly, biaxially stretched film is preferable. The film can beobtained, for instance, by extruding a polymer through a T-die extrusionmethod on a cast drum to prepare an unstretched film, stretching theunstretched film with a group of heated rollers in the machinedirection, feeding the film to a tenter or the like, and furtherstretching it in the cross direction. Unstretched film having a desiredthickness can be prepared by adjusting the slit width of an orifice,amount of a polymer to be extruded, and the number of revolution of acast drum. The film can be stretched at a desired stretching ratio byadjusting the rotational speed of a group of heated rollers and changingthe preset width of a tenter.

Flame retardant, thermal stabilizer, antioxidant, UV absorber,antistatic agent, pigment, dye, organic lubricant such as aliphatic acidester and wax, anti-foaming agent such as polysiloxane, and the likescan be added to the thermoplastic resin film, when necessary.

Thickness of the thermoplastic resin film is suitably decided dependingon the sensitivity and others required of the stencil sheet, and usuallyadjusted to 0.1 to 10 μm, preferably 0.1 to 5 μm, and more desirably 0.1to 3 μm. When thickness of the film exceeds 10 μm, perforatabilitysometimes reduces, but when it is less than 0.1 μm, the film formingstability occasionally lowers.

As the porous support used in the present invention and mainly composedof synthetic fibers, a paper prepared by a wet paper-making method,nonwoven fabric, woven fabric, or gauze screen from short fibers mainlycomprising synthetic fibers can be used, but the nonwoven fabric ispreferable.

As the synthetic fibers, known fibers of, for example, a polyester,polyamide, polyphenylenesulfide, polyacrylonitrile, polypropylene,polyethylene, or their copolymer is used. These synthetic fibers may beused in one kind of fibers, or two or more kind of fibers.Alternatively, the fibers may comprise natural fibers or regeneratedcellulose fibers. However, polyester fibers are preferable from theviewpoint of thermal stability at the time of perforation. Even when twoor more kind of fibers are used, the fibers preferably comprise at least60% by weight of polyester fibers.

As the polyester used for the synthetic fibers, polyethyleneterephthalate, polyethylene naphthalate, polycyclohexane-dimethyleneterephthalate, and copolymer of ethylene terephthalate with ethyleneisophthalate can be mentioned.

In these polymers, a flame retardant, thermal stabilizer, antioxidant,UV absorber, antistatic agent, pigment, dye, organic lubricant such asaliphatic acid ester and wax, and anti-foaming agent such aspolysiloxane can be blended, when necessary.

As the nonwoven fabric, one prepared by a known direct melt spinningmethod such as a flash spinning method, melt blow spinning method, andspun bond method is used.

For instance, the nonwoven fabric is prepared through a melt blowspinning method by blowing heated air from the circumference of aspinneret when a melted polymer is extruded through the spinneret tomake the extruded polymer into fine fibers by the heated air, and thencollecting the fibers on a net conveyer arranged at a suitable positiontaking advantage of the air to form a web.

The web is sucked together with the heated air by a suction deviceprovided at the net conveyer, and collected before individual fibers arecompletely solidified. That is, the web is collected in the statewherein the fibers are melt-adhered to one another. Extent ofmelt-adhesion of the fibers can be adjusted by suitably establish thedistance between the spinneret and the net conveyer. Besides, basisweight of the web and fiber diameter of a single fiber can be optionallyadjusted by properly controlling the amount of a polymer to be extruded,temperature of the heated air, flow rate of the heated air, and movingspeed of the conveyer.

Fibers spun by a melt blow method are thinned by the pressure of heatedair and solidified in not-oriented or low-oriented state. Thickness offibers is not uniform, and a web is formed in the state where thickfibers and thin fibers are properly dispersed. Polymer extruded from aspinneret is solidified in the state of a low crystallinity close toamorphous, since the polymer is rapidly cooled from a melted state to asolid state in the atmosphere at room temperature.

Further, in order to impart an affinity for a printing ink to a nonwovenfabric, the surface of fibers which constitute the nonwoven fabric maybe subjected to a chemical treatment by an acid, alkali, or the like,corona treatment, and low temperature plasma treatment, when necessary.

In the present invention, average diameter of fibers in the poroussupport is preferably 2 to 15 μm. When the average fiber diameter issmaller than 2 μm, wrinkles are liable to occur on a stencil sheet andunperforated portions are ready to be caused at the time of perforation,but when it exceeds 15 μm, unevenness is apt to be caused in passing ofthe ink.

Fiber basis weight (Metsuke) of the porous support is usually 2 to 30g/m², preferably 2 to 20 g/m², and more desirably 5 to 15 g/m². When thebasis weight exceeds 30 g/m², passability of an ink reduces anddefinition of images lowers, but when the basis weight is lower than 2g/m², a strength sufficient as the support sometimes can not beobtained.

Laminating of a thermoplastic resin film and porous support to integratethem can be performed by adhesion using an adhesive (binder) underconditions wherein sensitivity of the thermoplastic resin film toperforation is not reduced, or by adhering the thermoplastic resin filmand porous support under heating without using an adhesive.

As the adhesive, one of vinyl acetate type, acrylic type, vinylchloride-vinyl acetate copolymer type, polyester type, and polyurethanetype is used.

Also, as an adhesive of UV curing type, a mixture of a polyester typeacrylate, urethane type acrylate, epoxy type acrylate, or polyol typeacrylate with a photopolymerization initiator may be used. In this case,an adhesive comprising, as a main component, an urethane type acrylateis particularly preferable.

From the definition of printed images, it is preferable to directlyadhere a thermoplastic resin film and porous support by heat-adhesionwithout using an adhesive.

Heat-adhesion is usually performed by “heat-pressing” in which thethermoplastic resin film and porous support are directly stuck to eachother while being heated. While the method for the “heat-pressing” isnot specifically limited, heat pressing with a heated roller isespecially preferable from the viewpoint of processability.

In the present invention, it is particularly desirable to co-stretch anunstretched thermoplastic resin film and a nonwoven fabric in the statewherein they are adhered by heating. This heat-adhesion is preferablyperformed prior to the stage wherein a nonwoven fabric, and anunstretched film obtained by extrusion casting are stretched in themachine direction. Adhesion temperature is preferably between 80 and170° C. and more preferably 100 to 150° C.

A thermoplastic resin film and nonwoven fabric are integrated and cansatisfactorily be stretched without separating by co-stretching themunder a heat-adhered condition. A reticulated nonwoven fabric preferableas the support can be formed since fibers in the nonwoven fabric arestretched at this time in the state wherein they are melt-adhered to oneanother at their interlocking or connecting points.

Further, a thermoplastic resin film and porous support are directlyfixed to each other and can be integrated without using an adhesive bystretching them integratedly.

Method of co-stretching is not specifically limited, but a biaxialstretching is preferable, and it may be either a consecutive orsimultaneous biaxial stretching. In the case of a consecutive biaxialstretching, it is general to stretch in the order of machine directionand then cross direction, but it may be performed in the reverse order.

Stretching ratio is not specifically limited and it is properlyprescribed depending on the type of a thermoplastic resin to be used,perforation sensitivity required of a stencil sheet, and the like.However, about 2 to 8 times each in the machine direction and crossdirection are usually suitable. Besides, it may be restretched in themachine, cross, or simultaneously machine and cross directions, afterthe biaxial stretching. Further, it is preferable to heat treating thesheet after the biaxial stretching. Heat treating temperature at thistime is not specifically limited and it is properly selected dependingon the type of a thermoplastic resin to be used. However, a temperatureof 80 to 260° C. and a period of time of 0.5 to 60 seconds are usuallysuitable.

It is possible to stretch nonwoven fabrics in the state wherein manylayers of nonwoven fabrics having a different or the same fiber diameterand basis weight are piled up. Crystallinity of fibers in a nonwovenfabric is preferably 20% or higher and desirably 25% or higher inparticular. Relation between the melting point (Tm₁) of a thermoplasticresin film and the melting point (Tm₂) of a nonwoven fabric ispreferably Tm₁≦Tm₂.

Peeling strength of a laminated and integrated thermoplastic film andporous support is preferably 1 g/25 mm or higher, more preferably 3 g/25mm, and still more desirably 5 g/25 mm or higher. When the peelingstrength is lower than 1 g/25 mm, peeling of a thermoplastic resin filmfrom a porous support sometimes occurs when a stencil sheet is fed to aprinting apparatus.

In order to prevent sticking at the time of perforation, a releasingagent is preferably coated on the surface of the film which constitutesthe sheet in the present invention to form a layer of the releasingagent.

Coating of a releasing agent may be conducted at any one of a step priorto or after a biaxial stretching which is conducted after theunstretched film and unstretched nonwoven fabric described above wereheat-adhered, a step between the first stretching and second stretchingin a biaxial stretching, and a separate step after winding up. From theeffect of preventing sticking, it is especially preferable to coat priorto stretching. While a method for coating a releasing agent is notspecifically limited, it is preferable to coat by using a roll coater,gravuer coater, reverse coater, or bar (rod) coater.

As the releasing agent, a known agent comprising a silicone oil,silicone resin, fluorine type resin, or surface active agent can beused.

Various additives such as an antistatic agent, a heat resisting agent,an antioxidant, organic particles, inorganic particles, and a pigmentcan be mixed to the releasing agent. Further, various additives, forexample, a dispersing agent, surface active agent, antiseptic agent, anddefoaming agent may be added to a coating solution of the releasingagent for the purpose of improve the dispersibility.

Thickness of the layer of the releasing agent is preferably 0.005 to 0.4μm and more desirably 0.01 to 0.4 μm from the viewpoint of runnabilityof a stencil sheet at the time of perforation and stain resistance ofthe thermal head.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail with reference toExamples below. However, it should be understood that the scope of thepresent invention is by no means limited by such specific Examples. Inthe Examples, measurement and evaluation of various characteristics ofstencil sheets were carried out by the following methods:

(1) PPS Smoothness of Surface of Porous Support

PPS smoothness of the surface of the porous support of a sample stencilsheet was determined by employing a Parker Print-Surf Roughness TesterME-90 (produced by Messmer Buchel Co., Ltd.) at a clamping pressure of500 KPa using a hard-backing while a thermoplastic film is being pressedagainst the surface of the porous support of the sheet such thatwrinkles are not formed on the film. Determinations were carried out foroptional 5 portions of the sample, the average value of the 5determination values was calculated, and the average value thus obtainedwas assumed to be PPS smoothness of the surface of the porous support ofa sample stencil sheet.

As the thermoplastic film, the same film having a film thickness of 2 μmas that used for a stencil sheet was employed. In this time, PPSsmoothness of the film itself was 0.0 μm.

(2) KES Bending Rigidity B Value

KES bending rigidity B value of a sample heat-sensitive stencil sheetwas determined by employing a pure bending characteristic tester (JTC-1)(produced by Japan Seiki Seisakusho (precision machine manufacturing)Co., Ltd.) according to the following procedures:

First, a heat-sensitive stencil sheet was cut by a single-edged razor tocollect 10 sample sheets of 10 cm wide and 10 cm long. Next, a samplesheet was gripped with the distance between the stationary clamp of 20cm long and the movable clamp being 10 cm, and subjected to pure bendingin the range of curvature K=−2.5 to +2.5 (cm⁻¹) at a constant curvaturechanging rate of 0.1 (cm⁻¹/sec).

Relation between bending moment (bending torque) per unit length M(g·cm/cm) of a sample and curvature K (cm⁻¹) was plotted to obtain M-Kcurve shown in FIG. 4.

Inclination or slant (Bf) at a curvature K of between 0.5 and 1.5(K=0.5˜1.5) and the absolute value of the inclination (Bb) between −0.5and −1.5 were determined, and the bending rigidity B value per unitlength (g·cm²/cm) was calculated by using the following equation:

B (g·cm²/cm)=(Bf+Bb)/2

Average value of bending rigidity B value of 10 sample sheets wascalculated and the average value thus obtained was assumed to be KESbending rigidity B value.

(3) Wet Tensile Strength in the Machine Direction (Kgf/cm)

Heat-sensitive stencil sheet was cut by a single-edged razor to collect10 sample sheets of 15 mm wide and 150 mm long. Next, the sample sheetwas immersed in water so that the sheet became hydrophilic. The sheetwas stretched until it ruptured by employing a “Universal Tester:Autograph AGS-Type D” (produced by Shimadzu Corp.) at a testing rate of10 mm/min with the length of the test sheet being 100 mm, and the wettensile strength was calculated by dividing the load at the time of 2%(2 mm) elongation by the width of the sample sheet. Average value of thewet tensile strength of 10 sample sheets was calculated and the averagevalue thus obtained was assumed to be wet tensile strength in themachine direction.

(4) Average Fiber Diameter (μm)

Optional 10 portions of a nonwoven fabric in a sample heat-sensitivestencil sheet were photographed by an electron microscope (SEM) toobtain 10 photographs. Diameter of fibers in the 10 photographs weremeasured in such a manner that the diameter of optional 15 fibers perone photograph were measured to obtain diameter of 150 fibers in total,and the average fiber diameter was calculated.

(5) Fiber Basis Weight (g/m²)

Weight of a heat-sensitive stencil sheet was measured by a precisionbalance and converted into the weight per m². Weight corresponding tothat of the film was deducted from the converted weight and the weightthus obtained was assumed to be the fiber basis weight.

(6) Method for Evaluating Runnability of Sheet and Wrinkle Formation onSheet at the Time when Sheet is Wound Around Printing Drum

Heat-sensitive stencil sheet prepared was fed to a “Risograph” GR377(produced by RISO KAGAKU CORPORATION), run toward a printing drum whilebeing subjected to a blank perforation (printing ratio: 0%) and gridpattern perforation (printing ratio: 50%), and wound around the drum.

◯: Excellent

Δ: While small wrinkles occurred on the sheet, it was such a level thatthe sheet was practically usable for printing.

×: Wrinkles occurred on the sheet and the sheet was incapable of beingused.

(7) Method for Evaluating Elongation of Sheet at Printing

Heat-sensitive stencil sheet prepared was fed to a “Risograph” GR377(produced by RISO KAGAKU CORPORATION), a grid pattern perforation wasconducted, and then printing was carried out. Distance between optional2 points in the top to bottom direction in a printed paper was measured,and the degree of change of 1000th printed paper sheet to the firstprinted paper sheet was calculated.

⊚: Considerably excellent (Degree of change was lower than 0.1%)

◯: Good (Degree of change was 0.1% or higher but lower than 0.3%)

Δ: Practically usable level (Degree of change was 0.3% or higher butlower than 1.0%.)

×: Incapable of being used (Degree of change was higher than 1%.).

(8) Method for Evaluating Wrinkle Formation at Printing

Heat-sensitive stencil sheet prepared was fed to a “Risograph” GR377(produced by RISO KAGAKU CORPORATION), a grid pattern perforation wasconducted, and then printing was carried out. After 1000 paper sheetswere printed, conditions of the sheet on the printing drum were visuallyinspected and evaluated by the following criteria:

◯: No wrinkle occurred.

Δ: While minor wrinkle formation at printing was observed, the sheet wasin a level of practically usable.

×: Wrinkle formation at printing was observed and the sheet wasincapable of being used for printing.

EXAMPLE 1

Polyethylene terephthalate (η=0.60, Tm=254° C.) as a starting materialwas extruded by using a rectangular spinneret having 80 orifices eachhaving a diameter of 0.35 mm at a spinneret temperature of 285° C. by amelt blow method to form fibers, and the fibers thus formed werecollected on a conveyer while being dispersed to obtain a nonwovenfabric having a basis weight of 120 g/m² and average fiber diameter of12.0 μm.

Next, a copolyester (η=0.65, Tm=210° C.) comprising 85% by mole ofethylene terephthalate and 15% by mole of ethylene isophthalate as astarting material was extruded by using an extruder having a screwdiameter of 40 mm at a T die nozzle temperature of 270° C. and cast on acooling drum having a diameter of 300 mm to obtain an unstretched film.

The nonwoven fabric described above was placed on the unstretched film,fed to a heating roller, and pressed at a roller temperature of 80° C.to prepare a laminated sheet.

This laminated sheet was stretched 3.5 times in the flowing directionbetween heated rollers at 90° C., fed to a tenter type stretchingmachine and stretched 4 times in the width direction at 95° C., and heattreated in the tenter at 160° C.

Wax type releasing agent was coated on the surface of the film at theentrance portion of the tenter by using a gravure coater such that theweight of the releasing agent after drying became 0.1 g/m² to prepare aheat-sensitive stencil sheet.

The sheet thus obtained had a fiber basis weight of 11 g/m², averagediameter of fibers in the support of 6.0 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 1.52 μm, wet tensilestrength in the machine direction of the sheet was 310 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.059g·cm²/cm and 0.049 g·cm²/cm, respectively.

EXAMPLE 2

Heat-sensitive stencil sheet was prepared in the same manner as inExample 1 with the exception that a nonwoven fabric having a basisweight of 120 g/m² and average fiber diameter of 8.4 μm was used inplace of the nonwoven fabric in Example 1.

The sheet thus obtained had a fiber basis weight of 11 g/m², averagediameter of fibers in the support of 4.2 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 0.93 μm, wet tensilestrength in the machine direction of the sheet was 302 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.064g·cm²/cm and 0.051 g·cm²/cm, respectively.

EXAMPLE 3

Heat-sensitive stencil sheet was prepared in the same manner as inExample 1 with the exception that a nonwoven fabric having a basisweight of 80 g/m² and average fiber diameter of 12.2 μm was used inplace of the nonwoven fabric in Example 1.

The sheet thus obtained had a fiber basis weight of 7 g/m², averagediameter of fibers in the support of 6.1 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 1.58 μm, wet tensilestrength in the machine direction of the sheet was 203 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.025g·cm²/cm and 0.023 g·cm²/cm, respectively.

EXAMPLE 4

Heat-sensitive stencil sheet was prepared in the same manner as inExample 1 with the exception that a nonwoven fabric having a basisweight of 80 g/m² and average fiber diameter of 7.0 μm was used in placeof the nonwoven fabric in Example 1.

The sheet thus obtained had a fiber basis weight of 7 g/m², averagediameter of fibers in the support of 3.5 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 0.91 μm, wet tensilestrength in the machine direction of the sheet was 211 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.028g·cm²/cm and 0.024 g·cm²/cm, respectively.

EXAMPLE 5

Polyester film was provided by forming a single film in advance suchthat the film thickness became 1.7 μm at stretching.

Porous support prepared from natural fibers and synthetic fibers(polyester fibers) by using a wet papermaking process and having a basisweight of 10.5 g/m² was stuck on the film described above through apolyvinyl acetate resin in a coating amount of 0.8 g/m², and then asilicone type releasing agent was coated on the film surface to preparea heat-sensitive stencil sheet.

PPS smoothness determined when a thermoplastic film was pressed againstthe surface of the support of the sheet thus obtained was 1.74 μm, wettensile strength in the machine direction of the sheet was 350 gf/cm,and KES bending rigidity B value in the machine and cross directionswere 0.028 g·cm²/cm and 0.021 g·cm²/cm, respectively.

Comparative Example 1

Heat-sensitive stencil sheet was prepared in the same manner as inExample 1 with the exception that a nonwoven fabric having a basisweight of 60 g/m² and average fiber diameter of 13.0 μm was used inplace of the nonwoven fabric in Example 1.

The sheet thus obtained had a fiber basis weight of 5 g/m², averagediameter of fibers in the support of 6.5 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 1.63 μm, wet tensilestrength in the machine direction of the sheet was 154 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.021g·cm²/cm and 0.020 g·cm²/cm, respectively.

Comparative Example 2

Heat-sensitive stencil sheet was prepared in the same manner as inExample 1 with the exception that a nonwoven fabric having a basisweight of 80 g/m² and average fiber diameter of 6.0 μm was used in placeof the nonwoven fabric in Example 1.

The sheet thus obtained had a fiber basis weight of 7 g/m², averagediameter of fibers in the support of 3.0 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 0.72 μm, wet tensilestrength in the machine direction of the sheet was 210 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.030g·cm²/cm and 0.026 g·cm²/cm, respectively.

Comparative Example 3

Nonwoven fabric having a basis weight of 80 g/m² and an average fiberdiameter of 8.2 μm was prepared in the same manner as in Example 1.

Heat-sensitive stencil sheet was prepared in the same manner as inExample 1 with the exception that nonwoven fabric described above wasused and stretched in the flowing direction at 100° C.

The sheet thus obtained had a fiber basis weight of 7 g/m², averagediameter of fibers in the support of 4.1 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 0.93 μm, wet tensilestrength in the machine direction of the sheet was 232 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.017g·cm²/cm and 0.012 g·cm²/cm, respectively.

Comparative Example 4

Heat-sensitive stencil sheet was prepared in the same manner as inExample 1 with the exception that a nonwoven fabric having a basisweight of 120 g/m² and average fiber diameter of 6.0 μm was used inplace of the nonwoven fabric in Example 1.

The sheet thus obtained had a fiber basis weight of 11 g/m², averagediameter of fibers in the support of 3.0 μm, and a thickness of thethermoplastic resin film of 1.5 μm.

Further, PPS smoothness determined when a thermoplastic film was pressedagainst the surface of the support of the sheet was 0.71 μm, wet tensilestrength in the machine direction of the sheet was 340 gf/cm, and KESbending rigidity B value in the machine and cross directions were 0.070g·cm²/cm and 0.055 g·cm²/cm, respectively.

Characteristics of the sheets obtained by Examples 1 to 5 andComparative Examples 1 to 4 are shown together in Table 1. From Table 1,it can be understood that wrinkle formation on a heat-sensitive stencilsheet at the time of winding the sheet around a printing drum,elongation of the sheet at the time of printing, and wrinkle formationon the sheet at printing can simultaneously be avoided by using theheat-sensitive stencil sheet of the present invention.

TABLE 1 Wrinkle Fiber Average Film Wet KES bending PPS formation atWrinkle basis fiber thick- tensile rigidity smooth- winding Elonga-forma- weight diameter ness strength (g.cm²/cm) ness around print- tionat tion at (g/m²) (μm) (μm) (gf/cm) MD CD (μm) ing drum printingprinting Example 1 11 6.0 1.5 310 0.059 0.049 1.52 ∘ ⊚ ∘ Example 2 114.2 1.5 302 0.064 0.051 0.93 Δ ∘ ∘ Example 3 7 6.1 1.5 203 0.025 0.0231.58 ∘ ∘ ∘ Example 4 7 3.5 1.5 211 0.028 0.024 0.91 Δ Δ ∘ Example 5 10.5— 1.7 350 0.028 0.021 1.74 ∘ ⊚ ∘ Compara. 5 6.5 1.5 154 0.021 0.020 1.63x x x Example 1 Compara. 7 3.0 1.5 210 0.030 0.026 0.72 x x Δ Example 2Compara. 7 4.1 1.5 232 0.017 0.012 0.93 x ∘ ∘ Example 3 Compara. 11 3.01.5 340 0.070 0.055 0.71 X ∘ ∘ Example 4 Note: Criteria for EvaluationWrinkle formation at winding a stencil sheet around a printingdrum/wrinkle formation at printing: ∘: Excellent Δ: While small wrinklesoccurred on the sheet, it was such a level that the sheet waspractically usable for printing. x: Wrinkles occurred on the sheet andthe sheet was incapable of being used. Elongation of the sheet atprinting: ⊚: Considerably excellent (Degree of change was lower than0.1%) ∘: Good (Degree of change was 0.1 % or higher but lower than 0.3%)Δ: Practically usable level (Degree of change was 0.3 % or higher butlower than 1.0%.) x: Incapable of being used (Degree of change washigher than 1%.)

Since the heat-sensitive sheet for stencil printing of the presentinvention has a wet tensile strength in the machine direction of 200gf/cm or higher, KES bending rigidity B value in the machine or crossdirection of 0.02 g·cm²/cm, and a PPS smoothness at the porous supportside of the sheet of 0.9 μm or higher, wrinkles do not occur on thesheet at the time of winding the sheet around a printing drum,elongation of the sheet when a large number of paper sheets are printedis repressed, occurrence of wrinkles on the sheet at the time ofprinting can be prevented, and reproducibility of a manuscript inprinted paper sheets is excellent.

What is claimed is:
 1. A heat-sensitive sheet for stencil printingcomprising a laminate of a thermoplastic resin film and a porous supportmainly composed of synthetic fibers, the sheet having a wet tensilestrength in the machine direction of 200 gf/cm or higher, a KES bendingrigidity B value in the machine or cross direction of 0.02 g·cm²/cm orhigher, and a PPS smoothness determined when a film is pressed againstthe surface of the porous support of the sheet of 0.9 μm or higher. 2.The heat-sensitive sheet for stencil printing according to claim 1wherein the sheet has a wet tensile strength in the machine direction of300 gf/cm or higher.
 3. The heat-sensitive sheet for stencil printingaccording to claim 1 wherein the sheet has a KES bending rigidity Bvalue in the machine or cross direction of 0.03 to 0.10 g·cm²/cm.
 4. Theheat-sensitive sheet for stencil printing according to claim 1 whereinthe sheet has a PPS smoothness determined when a film is pressed againstthe surface of the porous support of the sheet of 1.1 to 8.0 μm.
 5. Theheat-sensitive sheet for stencil printing according to claim 1 whereinthe thermoplastic resin film is a polyester film.
 6. The heat-sensitivesheet for stencil printing according to claim 1 wherein thethermoplastic resin film has a thickness of 0.1 to 10 μm.
 7. Theheat-sensitive sheet for stencil printing according to claim 1 whereinthe porous support is a nonwoven fabric mainly composed of polyesterfibers.
 8. The heat-sensitive sheet for stencil printing according toclaim 1 wherein the average diameter of the fibers in the porous supportis 2 to 15 μm.
 9. The heat-sensitive sheet for stencil printingaccording to claim 1 wherein the porous support has a fiber basis weightof 2 to 30 g/m².
 10. The heat-sensitive sheet for stencil printingaccording to claim 1 wherein the sheet has a peeling strength of 1 g/25mm or higher.